The surfaces of most pelagic diatoms are sticky at times and may therefore form rapidly settling aggregates by physical coagulation. Stickiness and aggregate formation may be particularly adaptive in upwelling systems by allowing the retention of diatom populations in the vicinity of the upwelling center. We therefore hypothesized that upwelling diatom blooms are terminated by aggregate formation and rapid sedimentation. We monitored the de- velopment of a maturing diatom (mainly Chaetoceros spp.) bloom in the Benguela upwelling current during 7 d in February. Chlorophyll concentrations remained consistently high during the observation period (-500 mg Chl m ?) and phytoplankton grew at an average specific rate of 0.25 d-l. The diatoms were extraordinarily sticky, with stickiness coefficients of up to 0.40, which is the highest ever recorded for field populations. Combined with estimates of turbulent shear in the ocean such stickiness coefficients predict very high specific coagulation rates (0.3 d-l). In situ video observation demonstrated the occurrence of abundant diatom aggregates with surface water concentrations between 1,000 and 3,000 ppm. Despite the very high concentration of aggregates, vertical fluxes of phytoplankton were very low, with fractional losses CO.005 d-l, and the aggregates thus seemed to be near neutrally buoyant. Losses due to copepod grazing were also low (-0.025 d-l). Most of the aggregates were colonized by the heterotrophic dinoflagellate Noctiluca scintilluns that feed upon diatoms in the aggregates. The system appeared to be in near steady state; specific diatom growth rate, coagulation rate, and loss rate due to N. scintillans feeding were all of the same magnitude (0.25-0.3 d-l) and the latter two varied in concert. Our observations provide only partial support for the population retention hypothesis because aggregate buoyancy and N. scintillans grazing effi- ciently reduced the vertical flux of aggregates in this system.